1. FIELD OF THE INVENTION
The present invention relates to temperature sensing devices and more particularly to time-temperature patches for use with foodstuffs and the like.
2. DESCRIPTION OF THE PRIOR ART
The consumption of a galvanic cell with time and temperature is a well known phenomenon, particularly in the familiar zinc-carbon cell. Galvanic cells of this type have been extensively studied and the charge capacity of zinc is well known. Thus the consumption rate (coulombs) of the well known Leclanche cell is widely appreciated by those skilled in the art.
A Leclanche cell typically includes a zinc anode, a carbon cathode, and an ammonium chloride electrolyte with the positive and negative terminals connected to the carbon and zinc electrodes respectively. Since the charge capacity of zinc is well known (0.81 amp hours per gram) its consumption, through ionization, is also well known and predictable. Simply, a zinc electrode of a known mass provides a known number of atoms for ionic consumption and thus will consume itself in the course of producing current.
This galvanic consumption of a cell is well appreciated and has led to various developments including the time-temperature indicators described in U.S. Pat. No. 4,277,974 to Karr et al. These teachings, while suitable for their purpose, consume a single electrode as a combined function of temperature and time. Thus the foregoing indicator is primarily useful with foodstuffs which, like the cell, exhibit an exponential combination of time and temperature in their useful life. Since time and temperature are thus combined only a combined result is perceptible.
Unlike ordinary foodstuffs, wine and other liquors entail prolonged organic processes including residual fermentation and aging which ordinarily are a normal incident of production. Consequently, such products are not well suited for exponential indicators since a prolonged shelf life is a normal event of their use.
Nonetheless, such extended shelf life history is subject to its own constraints. Most frequently such constraints are in the form of upper temperature limits which, when exceeded, commence their deleterious effects on the product. An exponential galvanic cell under these circumstances will eventually consume itself, by the simple extent of its shelf life, and thus becomes useless as an indicator with time.
An indicator which is essentially rendered inactive under ordinary storage conditions is thus extensively sought and it is one such indicator that is disclosed herein.